Tire comprising a tread with rubbery foam

ABSTRACT

Tire comprising at least one carcass-type reinforcing structure anchored on each side of said tire in a bead whose base is intended to be mounted on a rim seat, each of said beads extending generally radially outwardly in the form of sidewalls, the sidewalls being connected radially towards the outside to a tread, the carcass-type reinforcing structure extending circumferentially from the bead towards said sidewall, a crown reinforcing structure, each of the beads also comprising an anchoring region for holding the reinforcing structure in place, wherein said tread comprises an inner layer of substantially incompressible rubbery material and an outer layer of compressible rubbery foam wherein:
         the foam expansion ratio is greater than 50%;   the thickness of the layer is greater than 2.0 mm and preferably greater than 4 mm; and   the outer layer has a Shore A hardness of less than 30 and preferably of between 10 and 20 Shore A.

The present invention relates to a tire with optimal grip and noisecharacteristics.

For some years now, tire manufacturers have been making strenuousefforts in order to develop original solutions allowing to obtain tireswith greater grip than has been achieved with conventional products.Usually this emphasis on performance is obtained at the cost of othercharacteristics of the tire. As a matter of fact, numerous technicalcharacteristics of this product are contradictory, with the conventionalconsequence that an improvement in one or two characteristics oftenleads to deterioration in other characteristics. Several performancetypes allow to illustrate this rule well. Grip is a good example.

To improve the grip performance of a product, it is conventionalpractice to use a two-layer tread configuration, with an outer layerwhose hardness is less than that of the inner layer. There are severaltypes of solutions allowing to obtain an outer layer whose hardness isless than that of the inner layer. Conventionally, for example, a layerof a material with a low Shore hardness creates significant constraintswhen it comes to efficient industrial use because it is difficult tohandle, and, because the material is particularly sticky. It adheres tothe tools and machines, which results in significant disruption of theprocess.

Document EP 0 873 884 describes a solution wherein the expansion ratioof the outer layer is between 1% and 100%. That document specifies thatan expansion ratio of greater than 100% has several major disadvantagessuch as reduced wear resistance and difficulty in producing a stableshape when the tire is manufactured.

Document FR 2 860 184 describes a foam tread whose stiffness is verylow, with expansion ratios of greater than 500%. This tread isconfigured to be used in tires for very heavy vehicles, in order tolimit damage to the ground. These machines also move at very slowspeeds.

To overcome these various disadvantages, the present invention providesa tire comprising at least one carcass-type reinforcing structureanchored on each side of said tire in a bead whose base is intended tobe mounted on a rim seat, each of said beads extending generallyradially outwardly in the form of sidewalls, the sidewalls beingconnected radially towards the outside to a tread, the carcass-typereinforcing structure extending circumferentially from the bead towardssaid sidewall, a crown reinforcing structure, each of the beads alsocomprising an anchoring region for holding the reinforcing structure inplace, wherein said tread comprises an inner layer of substantiallyincompressible rubbery material and an outer layer of compressiblerubbery foam wherein:

-   -   the foam expansion ratio is greater than 50%;    -   the thickness of the layer is greater than 2.0 mm and preferably        greater than 4 mm; and    -   the outer layer has a Shore A hardness of less than 30 and        preferably of between 10 and 20 Shore A.

Surprisingly, and contrary to expectation, it has been observed that thefoam layer does not cause a very significant deterioration in thebehavior of the tire. In fact, once crushed or sheared, the stiffnessreverts to acceptable levels. The time necessary for crushing is not aproblem for driving. Further, improvements are also observed in grip.The rate of wear achieved with such an architecture is moreover of anacceptable level, especially in urban usage. In any case, the slightdiminutions in performance in terms of behavior and wear are amplycompensated for by substantial improvements in grip.

Conventionally, layers of reduced hardness are obtained by using oiland/or plasticizers in the matrix of a material that is conventionallyrelatively hard. This method of reducing hardness has the drawback thatit makes the material sticky and difficult to process. The embodimentsof the invention do not use such agents. Instead, a modified structureof the material, comprising cells, is used.

The tire according to the invention is advantageously used for tiresintended for mounting on passenger cars or vehicles.

Conventionally, a layer of a material with a low Shore hardness createssignificant constraints when it comes to efficient industrial usebecause it is difficult to handle, and, because the material isparticularly sticky. It adheres to the tools and machines, which resultsin significant disruption of the process. By using, in accordance withthe invention, a cellular structure which allows the hardness to bereduced by a mechanical or structural effect rather than by using alayer of a material whose hardness is reduced with chemical agents,these problems are solved. The presence of a layer of a compressiblematerial means that as the tire rolls, it envelops the asperities of theground. This phenomenon contributes to a reduction in local impacts anda better distribution of the pressure forces.

The noise reduction achieved with the invention is around 3 to 4decibels or even more. This reduction is moreover spread over a verywide frequency spectrum (for example, between 50 and 800 Hz in sometests). Moreover, the noise reduction concerns both noise transmittedthrough solid objects (through the structure of the vehicle, up to about500 Hz) and noise transmitted by the air (from around 500 Hz up).

This reduction is due to the use of an outer layer with a reducedmodulus.

The stiffness of the matrix of the outer layer is preferably greaterthan 55 Shore A and preferably greater than 65 Shore A.

According to one advantageous embodiment, the tire comprises anintermediate layer situated between the outer layer and the inner layer,the intermediate layer having an expansion ratio less than that of theouter layer.

Said intermediate layer preferably has an expansion ratio of at least20% and preferably at least 50% less than that of the outer layer.

According to one advantageous variant, the thickness of the outer layeris less than the depth of the tread blocks of the tire.

According to another variant, the thickness of the outer layer isgreater than the depth of the tread blocks of the tire.

Advantageously, the outer layer consists mainly of closed cells. Saidcells are preferably not pressurized.

The compositions of the rubbery materials of the inner layer and of thematrix of the outer layer are advantageously identical.

According to one advantageous variant, the tire comprises a region ofprotection of the borders of the outer layer with at least one materialwhose expansion ratio is less than 10% and preferably less than 2% andwhose thickness is greater than 0.5 mm.

According to another advantageous variant, the tire comprises a regionof protection of the borders of the outer layer with the addition of atleast one bielastic fabric, such as Lycra (registered trade mark). Sucha fabric can be used anywhere outside of the outer layer, such as forexample on the surface of the tread). Such a fabric has the advantagethat it conforms fully to the tread blocks during the molding stage.

Protective components of this kind are very effective at protecting theouter layer in the event of a serious skid or drift.

All the details of construction are given in the following description,completed by FIGS. 1 to 6, in which:

FIG. 1 shows an illustrative embodiment of a tire according to theinvention seen in transverse section

FIG. 2 is an enlarged view of a portion of a crown of a conventionaltire of known type

FIGS. 3 to 6 are enlarged views of a portion of a crown of the tire withvarious variants

In the present description, “sidewalls” means those portions of thetire, usually of low flexural stiffness, situated between the crown andthe beads. “Sidewall compound” means the rubbery compounds situatedaxially outwardly relative to the threads of the reinforcing structureof the carcass and to their bonding rubber. These compounds usually havea low modulus of elasticity.

“Bead” denotes that portion of the tire which is radially inwardlyadjacent to the sidewall.

It should be pointed out that “radially upward” or “radially upper” or“radially outward” means towards larger radii.

The expression “based on” is used in the conventional way to mean thatthe constituent that follows is the majority constituent of thecomposition.

A carcass-type reinforcing or reinforcement structure will be describedas radial when its threads are at 90°, but also, following currentterminological usage, at an angle close to 90°.

Conventionally, as presented in FIG. 1, the tire comprises acarcass-type reinforcing or reinforcement structure 15 provided withreinforcements advantageously configured in a substantially radialarrangement. This structure can be arranged continuously from one beadto the other, passing through the sidewalls 2 and the crown 3 of thetire, or alternatively the structure may comprise two or more partsarranged for example along the sidewalls, without covering the whole ofthe crown.

Two main types of anchoring 4 for the carcass-type reinforcing structureare possible. Typically, as shown in FIG. 1, the turn-up of saidstructure 15 around a core 7 of the bead provides the anchoring 4 of thecarcass-type reinforcing structure in the bead.

Alternatively, the anchoring function can be provided by an arrangementof circumferential threads disposed in the vicinity of the reinforcingstructure 15. Circumferential threads arranged preferably in the form ofstacks form an arrangement of anchor threads in each bead. These threadsare preferably metallic, and may be brass-coated. Various variantsadvantageously employ textile threads, such as aramid, nylon, PET, PEN,hybrid or other threads, e.g. glass fibers. In each stack the threadsare advantageously basically concentric and superposed.

FIGS. 3 to 6 show various examples of how the invention may be carriedout. These various examples show how a layer of foam 6 can be positionedin the crown region of the tire. The crown thus comprises an inner layer8, or rubbery crown compound, consisting of a substantiallyincompressible rubbery layer, and an outer layer 6 consisting ofcompressible rubbery foam. The degrees ofcompressibility/incompressibility are such that the degree ofcompressibility of the outer layer 6 is substantially greater than thatof the inner layer 8.

The region of the crown covered by the foam layer advantageouslycomprises the tread, as shown in FIG. 1. This region may, in accordancewith various alternatives, be wider or narrower than the tread.

The rubbery foam used for the outer layer 6 preferably has an expansionratio of more than 50%. The thickness of the outer layer 6 isadvantageously greater than 2.0 mm and preferably greater than 4 mm.

The outer layer 6 has a Shore A hardness of less than 30 and preferablyof between 10 and 20 Shore A. The stiffness of the matrix of the outerlayer is preferably greater than 55 Shore A and preferably greater than65 Shore A.

It is preferable to use compositions of similar rubbery materials tomake the inner layer 8 or rubbery crown compound on the one hand, andthe matrix of the outer layer 6 on the other. The distinct properties ofthe outer layer are obtained by specific forming operations on thematerial in the form of foam.

This rubbery foam preferably consists mostly of closed cells. Thesecells are preferably not pressurized.

In the example shown in FIG. 3, the thickness of the foam layer is lessthan the depth of the tread blocks 5 of the tire. It will thus beobserved that the tread grooves or channels 9 continue radially inwardsfurther than the thickness of the outer layer 6.

In the example shown in FIG. 4, the thickness of the outer layer isapproximately the same as the depth of the channels 9. Lastly, in theexample shown in FIG. 5, the thickness of the outer layer 6 is greaterthan the depth of the channels 9. The channels are therefore entirelysituated within the foam. Such an arrangement avoids areas of stressconcentration and the risk of cracking between inner and outer layers.The thickness of foam beneath the channels is preferably less than 1 mm.

In the example shown in FIG. 6, the crown of the tire has anintermediate layer 10 between the outer layer 6 and the inner layer 8,the intermediate layer having an expansion ratio less than that of theouter layer. In such an example, said intermediate layer may have anexpansion ratio of at least 20% and preferably at least 50% less thanthat of the outer layer.

Lastly, and optionally, the crown region may be provided with a regionof protection of at least the borders of the outer layer 6 with theaddition of at least one bielastic fabric. Alternatively, there may alsobe a region of protection of the borders of the outer layer with atleast one material whose expansion ratio is less than 10% and preferablyless than 2% and whose thickness is greater than 0.5 mm.

The invention claimed is:
 1. A tire comprising at least one carcass-typereinforcing structure anchored on each side of said tire in a bead whosebase is intended to be mounted on a rim seat, each of said beadsextending generally radially outwardly in the form of sidewalls, thesidewalls being connected radially towards the outside to a tread, thecarcass-type reinforcing structure extending radially from the beadtowards said sidewall, the tire further comprising a crown reinforcingstructure, each of the beads also comprising an anchoring region forholding the carcass-type reinforcing structure in place, wherein saidtread comprises an inner layer of substantially incompressible rubberymaterial and an outer layer of compressible rubbery foam wherein: thefoam expansion ratio is greater than 50%; the thickness of the layer isgreater than 2.0 mm; and the outer layer has a Shore A hardness of lessthan 30 Shore A wherein the tire is adapted for mounting on passengercars.
 2. The tire of claim 1, wherein the stiffness of the matrix of theouter layer is greater than 55 Shore A and preferably greater than 65Shore A.
 3. The tire of claim 1, wherein the compositions of the rubberymaterials of the inner layer and of the matrix of the outer layer areidentical.
 4. The tire of claim 1, wherein the outer layer consistsmainly of closed cells.
 5. The tire according to claim 4, wherein saidcells are not pressurized.
 6. The tire of claim 1, comprising anintermediate layer situated between the outer layer and the inner layer,the intermediate layer having an expansion ratio less than that of theouter layer.
 7. The tire of claim 6, wherein said intermediate layer hasan expansion ratio at least 20% and preferably at least 50% less thanthat of the outer layer.
 8. The tire of claim 1, wherein the tirecomprises tread blocks and wherein the thickness of the outer layer isless than the depth of the tread blocks of the tire.
 9. The tire ofclaim 1, wherein the tire comprises tread blocks and wherein thethickness of the outer layer is greater than the depth of the treadblocks of the tire.
 10. The tire of claim 1, comprising a region ofprotection of at least the borders of the outer layer with the additionof at least one bielastic fabric.
 11. The tire of claim 1, comprising aregion of protection of the borders of the outer layer with at least onematerial whose expansion ratio is less than 10% and whose thickness isgreater than 0.5 mm.
 12. The tire of claim 1, wherein the thickness ofthe layer is and greater than 4 mm.
 13. The tire of claim 1, wherein theShore A hardness is between 10 and 20.